Well, not always. It began in the mid 90s when artists in anime could actually draw a girl more than just a stick figure with B cup boobs. And grew exponentially after Love Hina with the adoption of digital animation.

Now its getting even bigger thanks to studios like XEBEC and Production IG taking bigger chances and focusing on even more ambitious designs. But a 'healthy' anime girl was practically unknown in the 1980s and before. You basically only had three body types. Skinny, more skinny and loli.